Making a file "read only" by setting Mac permissions doesn't affect the file if it's emailed, only affects the file if it's read on a Mac, is defeated by changing the permissions or copying the file to a disk that has permissions disabled (by default, external drives have permissions disabled), and doesn't prohibit copying the contents of the file to the clipboard.
PDF files can be password-protected, and the password-protected PDF can be limited in many ways, including setting permissions for copying, modifying, and printing. However, there are programs circulating the Internet which strip the password from protected PDFs.
Really, the only realistic protections are legal. If you have a copyrighted PDF and your client steals it without paying you, sue them. if you can't afford to sue and you don't trust your client to pay you, drop that client and do business with clients you DO trust.
The above points are on point. So are several others made throughout this thread. Once the file is out of your hands, all bets are off.
For a bit of legal protection, make sure that you send/email yourself a copy of the file emailed to client — essentially a legal receipt for the mailing. Then, if necessary, you turn everything over to your lawyer/solicitor.
